Question

Thinking in terms of evolutionary advantage, why does the ventilatory response to hypoxemia not show an increase until the PaO2 decreases below 60 mmHg?

Answer 1

Yes it is true that only after the pao2 goes below 60 mmHg the chemoreceptors will be stimulated due to hypoxia.

This is because when the partial pressure of oxygen is above 60 mmHg there will be still enough blood in the artery to maintain the oxygen supply to tissue but when it goes below 60 mmHg the spo2 become less that 80% leading to reduction oxygen in blood for metabolism process that is hypoxia . This is the situation where chemoreceptors are stimulated.

So as artery has a good spo2 even when pco2 is 61mmhg or above is the reason why stimulation does not occur until then